Carbon nitride heterojunction and preparation method and application thereof

Abstract

The application discloses a carbon nitride heterojunction and a preparation method and application thereof. The carbon nitride precursor is subjected to an ionothermal polymerization reaction in the presence of a eutectic salt to prepare the carbon nitride heterojunction, and the carbon nitride heterojunction comprises polyheptazine imide and polytriazine imide, and a heterojunction is formed between the polyheptazine imide and the polytriazine imide. The application solves the problems of low interface matching degree and poor crystallinity of the carbon nitride heterojunction photocatalyst. The carbon nitride heterojunction prepared by the application has the advantages of high crystallinity and strong carrier transfer capacity, and can maximize the photocatalytic activity, and is expected to be applied to large-scale photocatalytic decomposition of water to produce hydrogen, photocatalytic reduction of carbon dioxide, photocatalytic nitrogen fixation and photocatalytic organic synthesis and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photocatalytic materials, in particular to a carbon nitride heterojunction and a preparation method and application thereof. BACKGROUND

[0002] Polymeric carbon nitride is a kind of photocatalyst without metal, which opens up a road for carbon nitride photocatalyst due to its high thermal stability, low cost and unique electronic band structure. However, at present, carbon nitride has the defects of high carrier recombination rate, slow migration rate and narrow light response range, and it is a difficult problem for researchers to improve the photocatalytic performance and stability of carbon nitride. Constructing a heterojunction is an extremely effective method to improve the photocatalytic performance and quantum yield. Due to the different energy band properties of different materials, a space potential difference will be formed on both sides of the heterojunction interface. The existence of this space potential difference is beneficial to the separation of photo-generated electrons and holes, so that the construction of a heterojunction can improve the photocatalytic activity and quantum efficiency of the material. However, the construction of a heterojunction requires extremely strict conditions, such as atomic-level interface contact and interface matching. Assuming that a crystalline carbon nitride and other photocatalysts (such as titanium dioxide, cadmium sulfide, etc.) heterojunction is constructed, due to the large difference in crystal structure between the two phases, it is difficult to achieve atomic-level interface contact. Polyheptazine imine (PHI) and polytriazine imine (PTI) are polymorphs, and the difference in crystal structure is small, and the electronic band structure has certain difference, which provides good conditions for synthesizing carbon nitride heterojunction with high interface matching degree. Previously, some researchers synthesized carbon nitride heterojunction by a two-step method, first synthesized PHI in the heterojunction, then added other eutectic salts, and synthesized PTI, the other phase of the heterojunction, on the basis of the existing PHI. However, the two-step synthesis method is not easy to achieve atomic-level contact, and lattice mismatch is easy to occur, and the interface matching degree of the heterojunction is low. When the lattice mismatch is serious, dislocations and recombination centers will occur, and the photocatalytic performance will be greatly reduced. In addition, poor crystallinity often leads to a large number of structural defects, which will act as recombination sites for electrons and holes, greatly reducing the quantum efficiency of the photocatalyst.

[0003] Therefore, the prior art still needs to be improved and developed. SUMMARY

[0004] In view of the deficiencies of the prior art, the purpose of the present application is to provide a carbon nitride heterojunction and a preparation method and application thereof, aiming at solving the problems of low interface matching degree and poor crystallinity of the existing PHI / PTI carbon nitride heterojunction.

[0005] In a first aspect of the present application, a preparation method of a carbon nitride heterojunction is provided, wherein the method comprises the following steps:

[0006] providing a carbon nitride precursor and a eutectic salt;

[0007] The carbon nitride precursor is subjected to an ionothermal polymerization reaction in the presence of the eutectic salt to obtain a carbon nitride heterojunction;

[0008] The carbon nitride heterojunction comprises polyheptazine imide and polytriazine imide, and a heterojunction is formed between the polyheptazine imide and the polytriazine imide, and the carbon nitride heterojunction is also referred to as a PHI / PTI carbon nitride heterojunction.

[0009] Optionally, the carbon nitride precursor is one of urea, cyanamide, dicyanamide, thiourea, and melamine.

[0010] Optionally, the eutectic salt is at least two of potassium chloride, sodium chloride, and lithium chloride.

[0011] Optionally, the mass ratio of the carbon nitride precursor to the eutectic salt is 10:5-10:6.

[0012] Optionally, the ionothermal polymerization reaction is performed at a temperature of 400-650 DEG C.

[0013] Optionally, the ionothermal polymerization reaction is performed for 3-12 hours.

[0014] In a second aspect, the present application provides a carbon nitride heterojunction, which is prepared by the method for preparing a carbon nitride heterojunction described above.

[0015] In a third aspect, the present application provides a use of the carbon nitride heterojunction as a photocatalyst.

[0016] Optionally, the use comprises one of photocatalytic decomposition of water to produce hydrogen, photocatalytic reduction of carbon dioxide, photocatalytic nitrogen fixation, and photocatalytic organic synthesis.

[0017] The present application uses a carbon nitride precursor to perform an ionothermal polymerization reaction in the presence of a eutectic salt to obtain a PHI / PTI carbon nitride heterojunction with high crystalline atomic-level contact. On the one hand, the PHI / PTI carbon nitride heterojunction is synthesized by a one-step method, and the two-phase PHI and PTI of the heterojunction are simultaneously generated under the ionothermal polymerization reaction of the two eutectic salts, which improves the probability of atomic-level contact of the heterojunction, reduces the lattice mismatch density, reduces the generation of dislocation defects, significantly reduces the recombination center, and improves the transfer and separation ability of the electron-hole pair, thereby improving the apparent quantum efficiency of photocatalytic hydrogen production. On the other hand, the ionothermal polymerization reaction of the two eutectic salts accelerates the thermal polymerization process, improves the polymerization degree and crystallinity of the PHI / PTI carbon nitride heterojunction, and reduces the hydrogen bonds in the carbon nitride layer, thereby reducing the recombination center in the semiconductor material and improving the separation efficiency of the photo-generated electron-hole pair, thereby significantly improving the photocatalytic activity. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 A schematic diagram for preparing PHI / PTI carbon nitride heterojunction in the embodiment 1 of the present application.

[0019] Figure 2 a, b and c are respectively the steady-state photoluminescence spectrum, the transient photocurrent spectrum and the electrochemical impedance spectrum of the PHI / PTI carbon nitride heterojunction prepared in the embodiment 1 of the present application.

[0020] Figure 3 a, b, c and d are respectively the SEM (scanning electron microscope) image, the TEM (transmission electron microscope) image, the TEM local image and the FFT (fast Fourier transform) image of the PHI / PTI carbon nitride heterojunction prepared in the embodiment 1 of the present application.

[0021] Figure 4 a, b and c are respectively the water splitting hydrogen production performance diagram greater than 420 nm, the quantum efficiency diagram under simulated sunlight and the quantum efficiency diagram under different wavelengths of the PHI / PTI carbon nitride heterojunction prepared in the embodiment 1 of the present application. DETAILED DESCRIPTION

[0022] The present application provides a carbon nitride heterojunction and a preparation method and application thereof. In order to make the purpose, technical scheme and effect of the present application more clear and explicit, the present application is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present application and not to limit the present application.

[0023] The existing PHI / PTI carbon nitride heterojunction has low lattice matching degree, poor crystallinity, weak carrier separation and transmission ability, low quantum efficiency and cannot meet the minimum requirement for solar-hydrogen energy conversion efficiency. Based on this, the inventors found that the PHI / PTI carbon nitride heterojunction prepared by one-step method can realize atomic-level interface contact, reduce the lattice mismatch constant and reduce the dislocation defects of the contact interface; the eutectic salt ion thermal polymerization reaction can accelerate the polymerization process, improve the polymerization degree and crystallinity of carbon nitride, reduce the hydrogen bond in the carbon nitride layer, reduce the recombination center in the semiconductor material, has high separation efficiency of photo-generated electron-hole pairs, the quantum efficiency reaches 48%, and the photocatalytic activity is significantly improved.

[0024] Specifically, the embodiment of the present application provides a preparation method of a carbon nitride heterojunction, which comprises the following steps:

[0025] providing a carbon nitride precursor and a eutectic salt;

[0026] performing ionothermal polymerization reaction on the carbon nitride precursor in the presence of the eutectic salt to prepare a carbon nitride heterojunction;

[0027] The carbon nitride heterojunction includes polyheptazine imide and polytriazine imide, and a heterojunction is formed between the polyheptazine imide and the polytriazine imide;

[0028] The molecular structure of the carbon nitride heterojunction is:

[0029]

[0030] The PHI / PTI carbon nitride heterojunction with high crystallization and atomic-level contact is prepared by ionothermal polymerization of a carbon nitride precursor in the presence of a eutectic salt. In one aspect, the PHI / PTI carbon nitride heterojunction is synthesized by a one-step method. The two phases of PHI and PTI are generated simultaneously in the ionothermal polymerization of the two eutectic salts, which improves the probability of atomic-level contact of the heterojunction, reduces the lattice mismatch density, reduces the generation of dislocation defects, significantly reduces the recombination center, improves the transfer and separation ability of the electron-hole pair, and thus improves the apparent quantum efficiency of the photocatalytic hydrogen production. In another aspect, the ionothermal polymerization is more complete in the presence of the two eutectic salts, which accelerates the thermal polymerization process, improves the polymerization degree and crystallinity of the PHI / PTI carbon nitride heterojunction, and reduces the recombination center of the PHI / PTI carbon nitride heterojunction due to the significant reduction of the intralayer hydrogen bond, thereby significantly improving the photocatalytic activity.

[0031] In one embodiment, the carbon nitride precursor is one of urea, cyanamide, dicyandiamide, thiourea, and melamine, but is not limited thereto.

[0032] In one embodiment, the eutectic salt is at least two of potassium chloride, sodium chloride, and lithium chloride, but is not limited thereto.

[0033] In the presence of the two eutectic salts, PHI and PTI can be generated simultaneously and the polymerization is more complete. By adjusting the proportion of the eutectic salt, the proportion of PHI and PTI in the PHI / PTI carbon nitride heterojunction can be controlled, thereby further improving the interface matching degree and crystallinity of the PHI / PTI carbon nitride heterojunction.

[0034] In one embodiment, the mass ratio of the carbon nitride precursor to the eutectic salt is 10:5-10:6, such as 10:5, 10:5.2, 10:5.6, or 10:6. Taking urea as the carbon nitride precursor and lithium chloride and potassium chloride as the eutectic salt as an example, the mass ratio of 10:5.2 has the optimal photocatalytic hydrogen production activity, and the mass ratio of potassium chloride to lithium chloride is 25:1.

[0035] In an embodiment, the temperature of the ionothermal polymerization reaction is 400-650℃, such as 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, etc. Taking carbon nitride precursor as urea and eutectic salt as lithium chloride and potassium chloride as an example, the optimal ionothermal polymerization reaction temperature for photocatalytic hydrogen production is 600℃.

[0036] In an embodiment, the time of the ionothermal polymerization reaction is 3-12h, such as 3h, 5h, 6h, 9h, 10h, 12h, etc.

[0037] The present application provides a carbon nitride heterojunction, wherein the carbon nitride heterojunction is prepared by the preparation method of the carbon nitride heterojunction as described above.

[0038] The present application provides an application of the carbon nitride heterojunction as a photocatalyst.

[0039] In an embodiment, the application of the photocatalyst includes one of photocatalytic decomposition of water to produce hydrogen, photocatalytic reduction of carbon dioxide, photocatalytic nitrogen fixation, and photocatalytic organic synthesis.

[0040] The present application is further described below through specific examples.

[0041] Example 1

[0042] The schematic diagram of the PHI / PTI carbon nitride heterojunction prepared by the one-step in-situ synthesis of the embodiments of the present application is shown in Figure 1

[0043] Taking 10g of urea as the carbon nitride precursor and 5g of potassium chloride and 0.2g of lithium chloride as the eutectic salt as an example, a PHI / PTI carbon nitride heterojunction (denoted as PHI / PTI-0.2) is prepared, and the specific preparation method is as follows:

[0044] 10g of urea, 5g of potassium chloride and 0.2g of lithium chloride are added to a 50mL crucible, the crucible is placed in a muffle furnace and heated at 600℃ for 3h, so that the urea and the potassium chloride and lithium chloride perform ionothermal polymerization reaction, and then naturally cooled to room temperature. The obtained carbon nitride heterojunction yellow powder is washed by suction filtration with deionized water. Finally, the PHI / PTI carbon nitride heterojunction yellow powder obtained after washing is vacuum dried at 60℃ for standby.

[0045] The characterization results of PHI / PTI-0.2 prepared in this example are shown in Figure 2 Figure 2 ​​In the figure a, b and c are respectively the steady-state photoluminescence spectrum, the transient photocurrent spectrum and the electrochemical impedance spectrum of PHI / PTI-0.2 and the carbon nitride PHI which does not form a heterojunction. Due to the construction of the heterojunction, the fluorescence intensity is weakened, which is caused by the charge transfer between the carbon nitride heterojunctions; the photocurrent intensity is increased and the Warburg radius is reduced, which indicates that the construction of the carbon nitride heterojunction strengthens the separation and transport ability of the carriers.

[0046] Figure 3 In the figure a, b, c and d are respectively the SEM image, the TEM image, the TEM local image and the FFT image of PHI / PTI-0.2 prepared in the example, from which it can be seen that the carbon nitride heterojunction realizes atomic-level contact and maintains very good crystallinity.

[0047] Example 2

[0048] The same as example 1, except that the amount of lithium chloride is changed to 0.1 g, 0.3 g, 0.4 g, 0.5 g and 1 g respectively, and the carbon nitride PHI / PTI heterojunctions thus prepared are denoted as PHI / PTI-0.1, PHI / PTI-0.3, PHI / PTI-0.4, PHI / PTI-0.5 and PHI / PTI-1 respectively.

[0049] Example 3

[0050] The photocatalytic decomposition of water to produce hydrogen reaction is as follows:

[0051] Take 50 mg of the carbon nitride PHI / PTI heterojunction prepared in example 1 and example 2 as a photocatalyst respectively, and take 50 mL of 10 vol% triethanolamine solution as a hole sacrificial agent solution, and transfer them into a quartz glass reaction tank, and ultrasonic treatment for 30 min. Then add chloroplatinic acid solution and load 3.0 wt% Pt nanoparticles as a cocatalyst under xenon lamp illumination. Use a full-automatic gas production glass system, under the conditions of 300 W xenon lamp and UVCUT-420 nm filter, determine the amount of hydrogen produced by an online gas chromatograph, so as to characterize the hydrogen production performance of the photocatalyst. Figure 4 In the figure a is the photocatalytic water splitting hydrogen production performance diagram of the carbon nitride heterojunction greater than 420 nm, from which it can be seen that the photocatalytic water splitting hydrogen production performance of the carbon nitride heterojunction is improved. Figure 4The photocatalytic activity of the carbon nitride heterojunction PHI / PTI-0.2 prepared in Example 1 is 255.9 μmol / h, and the hydrogen production activity of the carbon nitride heterojunction PHI / PTI-0.2 prepared in Example 1 is 2.8 times that of the carbon nitride PHI without forming a heterojunction. The photocatalytic activity of the carbon nitride heterojunction PHI / PTI-0.1 prepared in Example 2 is 192 μmol / h, the photocatalytic activity of the carbon nitride heterojunction PHI / PTI-0.3 is 202.1 μmol / h, the photocatalytic activity of the carbon nitride heterojunction PHI / PTI-0.4 is 190.7 μmol / h, the photocatalytic activity of the carbon nitride heterojunction PHI / PTI-0.5 is 81.1 μmol / h, and the photocatalytic activity of the carbon nitride heterojunction PHI / PTI-1 is 74.6 μmol / h.

[0052] Figure 4 The apparent quantum efficiency of the carbon nitride heterojunction PHI / PTI-0.2 with atomic-level contact under sunlight is 0.56%.

[0053] Figure 4 The apparent quantum efficiency of the carbon nitride heterojunction PHI / PTI-0.2 under different wavelengths is up to 48.34% at 420 nm.

[0054] In summary, the application discloses a method for one-step preparation of a high-crystalline carbon nitride heterojunction, which utilizes a carbon nitride precursor to form a high-crystalline carbon nitride heterojunction through ionothermal polymerization of a eutectic salt, and the proportion of the eutectic salt can be adjusted to control the proportion of PHI and PTI in the polymer carbon nitride, thereby solving the problems of low interface matching degree and poor crystallinity of the carbon nitride heterojunction. The obtained photocatalyst has the advantages of high crystallinity and high carrier separation and transport efficiency, and can improve the efficiency of conversion of sunlight into hydrogen energy.

[0055] It should be understood that the application of the application is not limited to the above examples, and those skilled in the art can make improvements or changes according to the above description, and all these improvements and changes shall belong to the protection scope of the appended claims of the application.

Claims

1. A method for preparing a carbon nitride heterojunction, characterized by, The method comprises the steps of: providing a carbon nitride precursor and a eutectic salt; carrying out an ionothermal polymerization reaction on the carbon nitride precursor in the presence of the eutectic salt to obtain a carbon nitride heterojunction; the carbon nitride heterojunction comprises polyheptazine imide and polytriazine imide, and a heterojunction is formed between the polyheptazine imide and the polytriazine imide; the carbon nitride precursor is urea; the eutectic salt is potassium chloride and lithium chloride; the mass ratio of the carbon nitride precursor to the eutectic salt is 10:5-10:6; the temperature of the ionothermal polymerization reaction is 600 DEG C; the time of the ionothermal polymerization reaction is 3 h.

2. Application of the carbon nitride heterojunction prepared by the preparation method of claim 1 as a photocatalyst.

3. Application of the carbon nitride heterojunction of claim 2 as a photocatalyst, which is photocatalytic decomposition of water to produce hydrogen.

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